The present invention generally relates to electronic circuits, and more particularly to a motor controller and to a method.
Switched Reluctance Motors (SRM) find more and more use, for example, in household appliances (e.g., washing machines, dishwashers, vacuum cleaners), industrial machines, and in cars (starter/generator, window lifter, pumps). The motor has a stator with consecutively switched windings (xe2x80x9ccoilsxe2x80x9d) creating a rotating magnetic field under control of a microcontroller and semiconductor switches.
For many applications it is desirable to know the actual mechanical position of the rotor in respect to the stator (rotor-to-stator angle). The position can be measured by a mechanical position sensor. However, the sensor would require space (inside or outside the motor), would be coupled by a extra cable (expensive, sometimes shielded), may limit the application scope of the motor (especially for high speed), and may exhibit further disadvantages (e.g., lower reliability). A sensorless position estimator using the microcontroller can be used instead.
A useful reference for construction and operation of a SPM with a sensorless estimator is: xe2x80x9cWolff, J.; Rahner, R.; Spxc3xa4th, H.: xe2x80x98Sensorless Speed Control of a Switched Reluctance Motor for Industrial Applicationsxe2x80x99, Proceedings of xe2x80x98Optimization of Electrical and Electronic Equipmentxe2x80x99, Vol. 2, p. 457-462, Brasov 1998xe2x80x9d Wolff et al. describe sensorless position estimation for an industrial motor (power supply 3xc3x97400 V/50 Hz) taking into account an assumed value R for the ohmic coil resistance. Compared to the coil voltage, the resulting ohmic voltage drop was small and could be have been neglected.
However, for low-voltage applications (e.g., 12 or 24 volts supply in a car), the coil resistance R becomes unneglectable. Resistance changes during the operation of the motor, for example, an 25% increase from start-up to continuous running duty when the motor is warming up, require a more accurate estimation. While it is possible to derive R for a motor stand-still in a calibration mode, there is a practical requirement to obtain R substantially at any time even if the motor is rotating.
The present invention seeks to provide an improved motor controller operating according to an improved resistance estimation method which mitigate or avoid these and other disadvantages and limitations of the prior art.